FIG. 1 shows a schematic depiction of a conventional copper access network which carries telephony and digital subscriber line (DSL) signals between an exchange building 210 and a plurality of customer premises 400. Each of the customer premises is connected to the local exchange building by a line 220, an example of which is shown in FIG. 1. The line comprises a pair of metallic conductors, normally made from copper. The line typically comprises an exchange portion 230, which connects the exchange to a primary node 235, and a distribution portion 240, which connects the primary node 235 to a secondary node 245. The secondary node 245 is connected to the customer premises 400 via a drop wire, which is often routed overhead via a telephone pole 255 but may be routed underground, for example in ducting. Further cable joints 265 may be made, for example to connect two lengths of cable together. It may be necessary to measure the electrical properties of a line at a cable joint, a primary node, a secondary node or at the customer premises.
It is common for network operators to be forced by regulators to open up their access networks for use by other communications providers, in a process known as local loop unbundling (LLU). In LLU, other communications providers (CPs) install their transmission equipment in local exchanges such that the CPs can offer telephony and/or broadband services to their customers. The applicant operates an access network to which over 400 CPs have access. The regulatory regime obliges the network operator to provide access to the network and to associated network services on an equivalent basis to both CPs and other divisions of the network operator's company. As a consequence of this, the minimum specification for the access network lines is defined so that CPs are able to plan the provision of their services. The specification for the applicant's access network is known as Supplier Information Note 349, SIN 349 (2009) (see also http://www.btwebworld.com/sinet/349v2p3.pdf).
The network operator needs to be able to measure the electrical parameters of the lines that comprise the access network to ensure that they are within the limits of the parameters defined in the specification. In order to do so, it will be understood that the measurement apparatus used should be calibrated such that CPs and others can be confident that the lines are within the specification.